The present invention relates, in general, to a mold clamping unit.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Vertical mold clamping units are used for opening and closing a mold and maintaining the mold in open and closed dispositions. Typically, operation of the mold clamping unit involves a conveyance of hydraulic oil under a certain pressure level between a hydraulic cylinder and a pressure accumulator, and between a hydraulic cylinder and a pump system. When the mold clamping unit is in open position of the mold, the weight force of the moved mass is partly absorbed by the cylinder piston of the hydraulic cylinder that is connected to the pressure accumulator, while the other part of the weight force is held by the hydraulic cylinder that is connected to the pump system. When, the mold is to be closed, the mold clamping unit has to overcome these pressures. In the hydraulic cylinder that is connected to the accumulator, an accumulator pressure acts on the fluid only in the lower pressure space. This pressure resultant from the pressure accumulator is preset and can normally not be influenced.
The pressure applied by the pump system for maintaining the mold clamping unit in open position is typically predefined heretofore by a preset pressurizing valve, normally a hand-operated pressure relief valve, which sets a specified limit for the back pressure. The term “back pressure” relates hereby to the pressure in the lower pressure space of the hydraulic cylinder that is connected to the pump system. This construction of the pressurizing valve is only able to operate at a fixed specified limit, whereby the limit must be chosen high enough to ensure a secure holding of the weight pressure of the platen even when the mold is fully opened. The operating pressure that builds up in the upper pressure space of the hydraulic cylinder that is connected to the pump must be chosen high enough to be able to overcome the back pressure as well as the pressure in the other hydraulic cylinder when closing the mold. This construction requires generation of a high operating pressure for closing of the mold clamping unit so that the power consumption of the hydraulic system is significant and causes in substantial energy consumption. Moreover, hydraulic components such as conduits, seals, pump, motor etc., must be dimensioned accordingly for such a high pressure.
It would therefore be desirable and advantageous to provide an improved mold clamping unit which obviates prior art shortcomings and which operates efficiently and at low pressures.